This application claims priority under 35 U.S.C. xc2xa7119(a) from Chinese Application No. 01118253.9, filed May 25, 2001.
The present invention relates to a method for preparing highly active electrocatalyst of proton exchange membrane fuel cells.
Proton exchange membrane fuel cell is a new type device for directly converting chemical energy into electrical energy. Due to the absence of rotating parts and combustion, the efficiency of energy conversion is not limited by Carnot cycle. By utilizing neat energy sources such as hydrogen, methanol and the like, no sulfur oxide and nitrogen oxide will be resulted, thus causing no environmental pollution. It is especially suitable to be used as movable electric sources and is useful for electric vehicles. With the advancement of technology, it gets more and more desirable to be industrialized. One of the main materials for such a device is the electrocatalyst, since its activity is directly related to the performance of the fuel cell. The catalyst produced by E-TEK Co. is one of the best commercialized catalysts having high ratio of activity to the amount of noble metals to be used. In laboratory, the catalyst is generally prepared by chemical reduction and the properties of the catalyst thus obtained depend significantly on the method of preparation. (1) Impregnation method is the most commonly used method for the preparation of supported metal catalyst (J. B. Goodenough, A. Hamnett, B. J. Kennedy, ETC. Elctrochimica Acta, Vol 15, No.1 pp. 199-207, 1990). The method comprises the step of putting the supporting material into a metal salt solution until thoroughly soaked, and then adding a reducing agent to reduce metal ions. This method takes advantage of capillary effect which enables the liquid to be diffused into the inner cavities of the supporting material. The active components dissolved in the liquid will then be adsorbed by the supporting material. Thus, the ability of the active components to be absorbed on the supporting material will play an important role on the property of the catalyst. (2) Metal vapor method (Wu Shihua, Yang Shujun and Wang Xukun et al., Petroleum Chem. Eng. (in Chinese) 18 (6), 361, 1989) comprises the step of vaporizing the metal, and then depositing the metal onto the surface of the supporting material. By this way, a catalyst with fine metal particulate and high activity is obtained. However, the method needs to use costly equipment and is difficult to produce on an industrial scale. (3) Redox method (Masahiro Watanabe, Makoto Uchida, Satoshi Motoo, J. Electroanal. Chem. 229 (1987) 395-406) comprises the step of forming a coordination compound from a metal ion and a ligand ion at reduced state, then adding an oxidant, an oxidized ligand and a metal ion to form a metastable solution, and finally adding the supporting material under suitable conditions to allow metal to be deposited on the supporting material. Catalysts thus prepared have uniform particle size and finer metal particulate. However, this method is time consuming. (4) Nanometer metal cluster synthesis method is a new method (T. J. Schmidt, M. Noeske, H. A. Gasteiger, R. J. Behm, J. Electrochem. Soc., Vol. 145, No. 3, March 1998), in which metal ions react with reducing agent in a suitable organic phase in the presence of stabilizer to form a nanometer metal cluster. Supporting material is then added to adsorb the formed metal clusters. Particle size of the metal catalyst prepared by this method is relative small, but the reaction condition of this method is quite critical. It is believed that a catalyst with excellent electrochemical activity could be obtained only when the platinum micro particles are non-crystalline and their sizes are about 4 nm (Masahiro Watanabe, Makoto Uchida, Satoshi Motoo, J. Electroanal. Chem. 229 (1987) 395-406). Owing to the adsorption equilibrium, the noble metals in the solution will firstly be reduced and the adsorption equilibrium will be shifted to the liquid phase. This results in desorption of the adsorbed noble metal. As a matter of fact, the main part of the noble metal is reduced in liquid phase. Coagulation of metal particles will definitely occur. As a result, homogeneity of the product decreases and the metal is poorly supported by active carbon.
The object of the present invention is to provide a method for preparing a nanometer electrocatalyst for proton exchange membrane fuel cells. By controlling the adsorption of noble metal by active carbon, noble metal catalysts with preferable particle size and crystalline state are obtained. The drawbacks of the above-mentioned methods are thus overcome.
There is provide a method for preparing a nanometer electrocatalyst for proton exchange membrane fuel cells, comprising the steps of:
1) adding in water a platinum halogen compound or a mixture of a platinum halogen compound and a ruthenium halogen compound, and active carbon, making the amount of noble metal in the solution in the range of 0.5-10 g/L, and the amount of active carbon in the range of 0.05-2 g/L;
2) adjusting the pH of the solution to 2.5-10.5 with potassium hydroxide and/or ammonium hydroxide;
3) adding dropwise a reducing agent to an amount of 2.5 to 5 times in excess of that of the noble metal in moles, and allowing the reduction reaction to proceed;
4) filtering off the liquid and washing the remains; and
6) drying the remains.
The platinum halogen compound and ruthenium halogen compound used in the method of the present invention is a compound containing platinum or ruthenium and halogen, such as platinum halide or ruthenium halide or their salt. The platinum or ruthenium ion may be a divalent or tetravalent ion. The compound may be a chloride, a bromide or an iodide. Preferably, the compound is a chloride, such as sodium chloroplatinate, sodium chlororuthenate, potassium chloroplatinate, potassium chlororuthenate, sodium chloroplatinite, sodium chlororuthenite, potassium chloroplatinite or potassium chlororuthenite. When a mixture of a platinum halogen compound and a ruthenium halogen compound is used, the molar ratio of platinum to ruthenium is 1:0.2-1.
The alkali used to adjust the pH of the solution of the raw materials is potassium hydroxide and/or ammonium hydroxide, and this is important to control the specific state of adsorption.
The reducing agent used in the method of the present invention may be aqueous solution of hydrazine hydrate, sodium borohydride, hydrogen and/or formic acid. The reduction reaction can be carried out at 0-70xc2x0 C., preferably at 50-65xc2x0 C., with a time of more than 15 minutes, preferably more than 30 minutes, and most preferably for about 1 hour. Preferably, the reducing agent is added dropwise with stirring.
When the reduction reaction is finished, lower the temperature of the solution to room temperature, filter off the liquid and wash the precipitate until no halogen ion can be detected. The product can be dried at 60-80xc2x0 C. in vacuum. By this way, a nanometer active carbon supported noble metal catalyst with particle size of 4xc2x10.5 nm can be finally obtained.
When the catalysts prepared by this invention were compared with those of E-TEK, it was found that the catalytic activity to oxidation reaction of methanol and hydrogen is significantly increased. It can be seen from the oxidation polarization curve that under identical oxidation reaction current density, the initial potential E for the methanol oxidation is 110 mV more negative, showing that the activation energy of the catalysts of the present invention to the methanol oxidation is significantly lowered. This provides a room for improvement of 110 mV on cell""s total working voltage. It was demonstrated that DMFC assembled with the catalyst of the present invention have a voltage 100 mV higher than that assembled with E-TEK catalysts under the same current density. The X-ray diffraction pattern of the crystals of the catalyst of the present invention and that of E-TEK demonstrated that they have a lower degree of crystallinity of metal platinum, and have lower and broader diffraction peaks of metal platinum. Lower degree of crystallinity means that the catalyst contains less platinum crystals and more active sites on the platinum surface. Therefore it has higher catalytic activity. High resolution electron microphotograph showed that the nanometer catalysts prepared by the present invention are non-crystalline with particle size in the range of 4xc2x10.5 nm.
The product obtained by the method of the present invention is specifically suitable for the preparation of catalysts for direct methanol proton exchange membrane fuel cell.